The present invention relates generally to the field of pressure relief units for mechanical refrigeration systems, and more particularly in the preferred embodiment to low pressure centrifugal chiller vessels that are protected from over-pressurization by a fragmentary rupture disk.
A low pressure centrifugal chiller vessel is generally utilized in commercial and industrial refrigeration systems, such as for providing air conditioning in hotels, cooling fluid for a manufacturing process, and commercial food refrigeration systems. A low pressure centrifugal chiller vessel generally operates under a vacuum of about sixteen inches of mercury, and should not operate at a pressure exceeding fifteen pounds per square inch above atmospheric pressure. Inherent to commercial and industrial refrigeration systems is an expectation that there will be a minimal amount of down time. Typically, these refrigeration systems are operated until a system breakdown occurs. At that time repair service is initiated to put the system back into operation. The down time that results from this kind of reactive maintenance program is, at best, an inconvenience for the system user. It often can have very costly consequences, such as food spoilage in commercial food refrigeration systems.
A mechanical refrigeration system including a low pressure centrifugal chiller vessel generally utilizes a chlorinated fluorocarbon (CFC) refrigerant. CFC refrigerants, many of which are sold by Dupont under the well known tradename FREON, have various boiling points, depending on the particular type of CFC refrigerant. Some typical types of CFC refrigerants are, for example R-11, R-113, and R-123. Freon and its related family of compounds are well known and widely used as heat transfer media in mechanical refrigeration systems.
Refrigeration systems generally include the pressurized storage of a vaporized refrigerant. To comply with applicable safety codes these systems have a pressure relief system for venting an over-pressurized storage vessel. For many years, it was the practice in the industry to design the pressure relief system to vent the CFC refrigerant from the over-pressurized storage vessel directly into the atmosphere. Recently, however, because of concerns for the environment and possible destruction of the ozone layer above the earth, it has become desirable, and in many cases mandated by law to minimize the release of CFC refrigerants into the atmosphere.
Environmental concerns, though significant, are not the only factor in favor of preventing the release of CFC refrigerant into the atmosphere. In recent years the cost of CFC refrigerants has escalated drastically, having increased over ten fold for some refrigerants in the past few years, while the available supply of CFC refrigerants is dwindling. For these reasons it is desirable to insure that no significant amount of CFC refrigerant is vented into the atmosphere through the pressure relief system.
In the design of mechanical refrigeration systems it has become an industry practice to provide a mechanical re-seating type relief valve. This valve is in series with and down stream from a fragmentary carbon rupture disk. The fragmentary carbon rupture disk is calibrated to burst into pieces at a predetermined maximum pressure. At any pressure up to the maximum pressure the carbon rupture disk provides an excellent positive seal to prevent the venting of CFC refrigerant into the atmosphere, and the infiltration of any contaminants into the refrigeration system. A combination of the fragmentary carbon rupture disk and the mechanical re-seating relief type valve, provides the positive seal characteristics of the carbon rupture disk and the re-seating capability of the mechanical relief valve.
A common limitation of refrigeration systems, having a mechanical re-seating relief valve downstream from a fragmentary carbon rupture disk is that the fragments from the burst carbon rupture disk often lodge in the seat of the relief valve. Naturally any debris or fragments of significant size that lodge in the seat will interfere with the re-seating of the mechanical relief valve after the refrigerant pressure has dropped below a predetermined threshold. Absent the positive seal provided by the mechanical re-seating relief valve, the CFC refrigerant is vented into the atmosphere.
Many designers of refrigeration systems have attempted to overcome this common limitation by utilizing a non-fragmentary metal rupture disk in place of the fragmentary carbon disk. The use of the non-fragmentary metal rupture disk has been shown to eliminate fragments from the burst rupture disk interfering with the re-seating of the mechanical relief valve. However, the metal rupture disk leaves unsolved a litany of other problems and creates a particularly undesirable consequence of its own. One problem the metal rupture disk leaves unsolved pertains to the interference with the re-seating of the mechanical relief valve by other contaminates occurring in the refrigeration system.
A consequence of retrofitting an existing carbon-disk refrigeration system with a non-fragmentary metal rupture disk is that the refrigeration system and the associated chiller vessel must be evacuated. In order to perform this task it is necessary to shut down the commercial or industrial operation utilizing the cooling capacity from the refrigeration system to capture the charge of the CFC refrigerant. This charge of CFC refrigerant can be from 200 lbs. to 2,500 lbs., or more. The evacuation of CFC refrigerant from the system is a very time consuming and costly procedure to perform. In addition, many commercial and industrial refrigeration systems are in constant demand and have no scheduled down time, therefore, the shutting down of the system decreases the production time of the associated commercial or industrial operation.
There remains a need for a system for eliminating any debris which could interfere with the re-seating of the mechanical pressure relief valve, and yet does not require the evacuation of the CFC refrigerant prior to installation. The present invention satisfies this need.